Polarization is one of the primary attributes of an optical field. Techniques for generating polarized light and measuring polarization states of light have been studied for decades. Conventional technologies usually work only at single respective wavelengths of light. For example, in a conventional full-Stokes division of a focal-plane polarimeter, a retarder layer and a linear polarizer layer are utilized in combination with a focal plane array. Different combinations of retarder and polarizer orientations transmit different respective polarization states to respective underlying imaging pixels. (For example, a macro-pixel usually comprises a 0°, a 45°, a right-hand circular, and a 90° polarizer.) However, retardance conferred by a single retarder layer is a function of wavelength. Therefore, a polarimeter comprising a conventional focal-plane array can sample polarization states only at single specific wavelengths within a limited band. These polarimeters have applications in a limited wavelength range and can require long exposure times because most of the light signals outside the wavelength range are filtered out.
U.S. patent application Ser. No. 13/287,910, (the “'910 application”) referenced above, discloses, inter alia, polarizers and methods for producing them. The polarizers comprise photopolymers having molecular orientations established by exposure to linear polarized radiation (e.g., linearly polarized UV light). For example, on a substrate surface a first alignment layer defines at least one respective alignment direction, wherein the first alignment layer comprises a respective photo-orientable polymer network (PPN). A first liquid crystal polymer (LCP) layer is situated on or proximate the first alignment layer so that the LCP (including at least one guest material) is aligned with the PPN. A first barrier layer is disposed on the first LCP layer to protect the underlying layers. These first layers comprise a first polarization layer group configured as, for example, a polarizer. A second alignment layer is proximate the first barrier layer, wherein the second alignment layer comprises a respective PPN defining a respective alignment direction. A second LCP layer is proximate the second alignment layer so that second LCP (with respective guest material) is aligned with the alignment direction(s) of the second alignment layer. A second barrier layer is disposed on the second LCP layer. The second layers comprise a second polarization layer group configured as, for example, a retarder. A polarizer device having such a multilayer structure can extend uniformly (e.g., over the surface of the substrate) or can be formed as a “pattern,” i.e., in multiple discrete zones on the substrate.
Light-polarizing devices are also described in the '991 application, of which an example comprises a substrate, a first polarization group of layers on the substrate, and at least a second group of polarizing layers. The groups of polarizing layers comprise respective alignment layers, LCP layers, and barrier layers, formed as summarized above.
Polarized-light emitters are also described in the '910 application, of which an example comprises a substrate and a first group of polarizing layers supported by the substrate. The first group comprises a respective alignment layer, a respective LCP layer, and a respective barrier layer. The LCP layer comprises a LCP material and one or more anisotropic fluorophores that are aligned with the alignment layer. A second polarization layer group defining a retarder can be situated relative to the substrate. Application of fluorescence-excitation energy, such as ultraviolet light or electrical current, causes the fluorophores to fluoresce. At least a portion of the fluorescent light can be transmitted through the LCP layer. At least a portion of the fluorescent light can be reflected back to the LCP layer in which the fluorescent light in a particular polarization state is absorbed by the aligned fluorophores.
Whereas the devices summarized above are useful for many applications, certain applications are unmet or poorly met by them. For example, there are current needs for polarizer-based devices that are operable over wider ranges of light wavelength; i.e., so-called “broadband” ranges. Example broadband devices include, but are not limited to, polarizers, polarimeters, retarders and other waveplates; polarized-light emitters, displays, and cameras.
As used herein, “broadband” means an ability to operate at a wide bandwidth that covers a useful range of the spectrum. For example, a broadband or achromatic wave plate can operate with flat retardance in a wavelength range of 260-410 nm, 400-800 nm, 690-1200 nm, or 1100-2000 nm. Other wavelength ranges are also possible. For such devices, the wavelength-dependence of the retardation is nearly flat (less than 0.05-0.1 wave deviation) over the entire operating wavelength range. In comparison, a narrow-band wave plate can operate in the range of, for example, 550-650 nm (centered at 600 nm) with a wavelength dependence of the retardation being less than 0.1 wave deviation across the operating wavelength range. It will be understood that operability in a broadband manner (e.g., in any of the broadband ranges noted above) generally includes operability in any of various narrower sub-ranges within the breadth of the broadband range. For example, a broadband device operable in the range of 260-410 nm is also operable in the range of 270-280 nm, which is a sub-range of the broadband range.
There are also current needs for polarizer-based devices that are operable over wider angles than currently available (or provided), particularly for improving displays. As used herein, “wide-angle” means viewing angle greater than 160 degrees. An example of the viewing angle of a twisted nematic liquid-crystal display (LCD) ranges from 160-170 degrees.
For many applications, such as light imaging, detecting, and display, there is a need for retarders, polarizers, and polarized-light emitters that can operate at multiple wavelengths and angles. For example, it would be useful if full-Stokes division-of-focal-plane (DoFP) polarimeters were available for operation at multiple wavelengths in the electromagnetic spectrum (e.g., in the visible spectrum (400-700 nm)) and at wider than conventional angles of incident light. In sensors, broadband operation would be useful for increasing the amount of light reaching individual sensor elements or for producing larger signals. Because the angle of incidence of incoming light rays on a sensor is affected by the locations of upstream optics, wider-angle operation would accommodate a greater range of lens focal-lengths and lens-object separations than currently. In displays, patterned sets of broadband retarders and polarizers would be useful for emitting light at multiple wavelengths at wide viewing angles in a patterned array such as an array of display pixels. This is because most displays need to operate in more than one color and must be visible at more than one angle.